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Abstract: As the global climate changes, the higher latitudes are seen to be warming significantly faster and it is likely – if not already apparent – that the Arctic biome will experience considerable shifts in ice melt season length and permafrost thawing, leading to changes in photoirradiance and in the freshwater and terrigenous inputs to the marine environment. The exchange of nutrients between Arctic surface and deep waters and their biogeochemical cycling throughout the water column is driven by the seasonality of some of the most extreme environmental changes on the planet. The impacts, however, of the current global climate transition period on the biodiversity and its continued nutrient cycling within the Arctic Ocean are not yet known. To determine seasonal variation in the microbial flora and fauna of the deep water column, samples were collected from a 1000m depth profile in the seas around the Western coasts of the Svalbard archipelago throughout the polar year. High-throughput sequencing of tag amplicon and shotgun metagenomes were used to monitor microbial diversity and function in both the epipelagic surface waters (defined by the diametric diurnal conditions of the polar summer and winter) and the relatively invariable and permanently dark mesopelagic depths.

In epipelagic surface waters ( 200m), seasonality subsequently had much less effect on biodiversity. Interestingly, species richness consistently increased down through the water column, with the deepest darkest waters containing the greatest diversity. The phenomenon of the polar phytoplankton blooms, followed by the successional explosion of heterotrophic bacterial populations, also seemingly spurs the annual disappearance of the ancient and chemolithoautotrophic marine Archaea from surface waters. During the winter darkness, these venerable microbes feast upon the summer fruits of the phytoplankton photosynthesis, replenishing surface waters with nutrients which fuel the next spring bloom, essentially yin to the phytoplankton’s yang.

However, should suggested models of a freshening Arctic be correct, surface Arctic basin waters in a warming world may become increasingly stratified, such that the vertical flux of nutrients between deeper waters and the epipelagic zone may be much reduced; primary productivity would consequently be lessened and this annual biogeochemical cycle, so essential for Arctic Ocean productivity, would inevitably be disrupted. Given the significance of the annual phytoplankton bloom pattern on prokaryote diversity in Arctic waters, any changes to bloom dynamics resulting from accelerated global warming will likely have major impacts on surface marine microbial communities, those impacts inevitably trickling down into deeper waters.